1. Field of the Invention
Embodiments herein relate to methods and systems for connecting packet-oriented communication terminals which are separated through a network address-translating unit and in which a gateway controls the transmission of a local communication terminal.
2. Background of the Art
Packet-oriented networks have been separated from other packet-oriented networks passing through them by placing a so-called “NAT device” or NAT unit (Network Address Translation) between networks. Such a NAT unit is found frequently at the boundary between a local network or LAN (Local Area Network) and a public network generally identified as the “Internet”.
The NAT unit enables the use of one set of IP addresses (Internet Protocol) for a LAN internal data packet exchange and another set of IP addresses for an external data packet exchange. In the case of an internal data packet exchange, private IP addresses are also referred to here, while in the case of an external data packet exchange, external IP addresses are referred to here.
The basic principle of NAT is the assignment (mapping) of each private IP address to a corresponding public IP address. This mapping is also called “binding” and is generally limited to a certain period of time.
A further embodiment of said NAT unit is known by the name NAPT (Network Address & Port Translation). In this case there is an additional reference of external port numbers to internal port numbers. NAT and NAPT are described in the publication RFC 3022 from the IETF (Internet Engineering Task Force).
The simplest implementation of NAT is modifying the source address of a data packet sent from the LAN. This process is called “Source NAT”.
In an incoming data packet, i.e., a data packet sent from a public network to the LAN, can have its destination address modified. This process is called “Destination NAT”. Typically, for correct mapping of the arriving data packets through the Destination NAT process, it is assumed that the NAT unit has a mapping table in which mapping or binding is recorded between the public address recorded in the header of the data packet and a private address to be mapped. Such binding assumes that, for the incoming data packet, an outgoing data packet was sent, based on whose source and destination addresses a binding was created. With the exception of a so-called “Full Cone NAT”, it is usually not possible to initiate a data packet exchange from outside of the LAN.
This is especially hindering for communication protocols in which a calling subscriber is attempting to reach a called subscriber in a LAN. When using communication protocols, e.g. SIP (Session Initiation Protocol) or H.323, problems regularly occur with NAT units.
A further problem emerges due to the fact that, although modifying the data packet through the NAT unit allows for a change in the address of the data heading entries or “Header” of the data packet, information at higher levels, in particular SIP, is not taken into account.
Methods have been reported to partially solve these problems. In particular, there are protocols called STUN (Simple Traversal of UDP through NAT), UPnP (Universal Plug and Play), TURN (Traversal Using Relay NAT) and ICE (Interactive Connectivity Establishment) which partially ensure the correct delivery of data packets.
These methods are dependent primarily on the implementation of a corresponding protocol stack in the participating communication terminals. In communication terminals, this involves packet-oriented communicating telephones—with differing processing performances depending on their designs—or computers with corresponding audio and/or video communication capabilities. Such communication terminals are commonly known as VoIP communication terminals (Voice over Internet Protocol) in the professional world.